Radio receiver



s. Y. WHITE RADIO RECEIVER June 30,1942.

Filed Feb. 29, 1940 W v K MS Y ATTORNEYS Patented June 30, 1942 UNITEDSTATES PATENT OFFICE RADIO RECEIVER Sidney Y. White, Wilmette, n1.Application February 29, 1940, Serial No. 321,377 9 Claims. (Cl. 250-40)This invention relates to a radio receiver adapted to insure positivecontact with a transmitter transmitting an unmodulated or modulatedcarrier wave and more particularly to the reception of speech modulatedcarrier waves having a frequency above 20 megacycles.

The diiiiculties of establishing positive contact between a receiver anda transmitter working at such high frequencies are well recognized andcontact is not easy to establish even when the carrier frequency is lefton, as in the present broadcasting practice. When the carrier is on onlywhile talking, as in some systems, the period of communication may bevery short, as for example when the entire message may comprise thesingle word yes, the diihculty of tuning in the receiver becomes evengreater. It is a broad object of the present invention to establishpositive contact with a high frequency transmitter by automaticallyscanning the portion of the frequency spectrum which includes thefrequency of the desired transmitter.

The amount of seaming coverage, namely, that percentage of thefrequencyspectrum which is covered by each passage of the automaticscanning device must include the zone of error" which includes thenecess zry tolerances both of the transmitter and the receiver involvedin communication of this type. These tolerances are made up of 1)thermal drift of tubes and other heated components of both transmitterand receiver, (3) voltage variations which affect the frequency, (3)errors in calibration of scales used on dials, etc., (4) personal errorsin making adjustments of tuning instrumentalities, etc. The total zoneof errorarising from such causes may be substantially 0.4% or thetransmission frequency at the present stage of the art. While undernormal circumstances .a scanning sweep, either manual or automatic, ofthis amount would establish positive contact between a transmitter andany receiver, by increasing the scanning range we may add to the zone ofpositive communication. By employing a total scanning range of 1% of thecarrier frequency, when receiving carriers in the region of 150megacycles, we may so arrange it that 0.2% is allocated at each end ofthe range to overcome the several errors making up the "zone of error."This leaves 0.6% free in the middle of the scanning zone in which othertransmitters may be located, all of which may be simultaneously operatedon different non-interfering frequencies. In the example illustrated, 6such transmitters could be employed whose carrier frequencies diifer bykilocycles, so that no interference would result between them if eachemployed the usual 10 kilocycle modulation band. It is an object of thepresent invention to provide such a receiver which will automaticallyscan a portion of the spectrum in which several transmitters may beoperating and to quickly provide positive contact with one of them andautomatically maintain such contact while the transmission continues.

Where the receiver is of the superheterodyne type, the scanning may beeffected by any means which varies the oscillator frequency and mayinclude such mechanical elements as motordriven variable condensers,movable ferro-magnetic cores or short-circuited rings. The motor couldbe a rotating device of any known type, a Rochelle salts crystal, acondenser of a telephone diaphragm type, etc. Where such a motor deviceis used, however, it is open to the serious objection that it hassubstantial inertia and it is practically impossible to stop it at theproper position to insure correct tuning and it also tends to hunt.According to the present invention, this objection is overcome byproviding a scanning device which has no moving mechanical parts and canbe made practically inertialess.

It is a further object of the invention to provide a means for use inassociation with such a scanning device which operates on passing over acarrier and is responsive to the frequency and not to the amplitude ofthe received wave. If it were responsive to the amplitude, it might passover several carriers and only stop on the strongest carrier. This wouldresult in delay in establishing communication between a receiver and oneof several transmitters operating simultaneously on differentfrequencies and all desiring to contact the receiver. According to thepresent invention, indicating means are provided to indicate the firstcarrier encountered and thus provide for a more rapid setting up of thecommunication channel.

According to a further object of the invention a director device isprovided which becomes immediately effective to control the tuning ofthe receiver once a carrier has been located by the scanning device.This director is arranged to overpower the scanning device so that it isnot necessary to stop the latter, which is left in operation. Thisfeature becomes important in designing receivers for communication andmobile work where rapid response of a receiver to a desired carrier isnecessary from the following sively on a plurality of carrierfrequencies ineither regular or random sequence. In each of these cases,when the carrier disappears, the receiver should instantly respond andcommence scanning again to pick up the carrier as soon as it lays down ausable field strength.

The desirability of not stoppin the scanning device arises from thenecessity of extremely rapid action in locating a carrier and from theproperty of scanning means requiring a rather long time to becomeoperative and to stop. The first point becomes apparent when it isconsidered that in the case of the generation of a half cycle wave by amulti-vibrator, it requires about seconds to build up to full amplitude.If it is shut off, as by disconnecting it from its source of platevoltage, large charges are left in its condensers which require asubstantial period of time to be dissipated through the very largeresistances employed. The present invention provides means whereby thescanning device is continually operative so that immediately upon thedisappearance of a carrier which has been tuned in, it is effective totune the receiver over its tuning range in its search for a new carrier.It is a further object of the invention that the time interval requiredto pick up the carrier be kept to a small value, preferably a fractionof a second.

The voltage generated by the scanning device may be of any desired formsuch as saw-tooth, square, sine, or triangular. The triangular waveshape is ideal in that the spectrum may be scanned steadily in onedirection up to the end and then instantly reversed and scanned in theopposite direction at the same uniform speed. There is some objection,however, to generators which generate this wave shape in that they. areusually bulky and heavy, as now known in the art.

The saw-tooth wave shape provides a steady sweep in one direction and asnap back return. This has the advantage that the carrier is alwaysapproached from one direction allowing the more effective use of a beatfrequency oscillator at the final detector. be an advantage in pickingup a preferred station toward the beginning of the sweep, so that if twostations appear simultaneously, the receiver will probably pick up thepreferred one.

The sine wave sweep obviously slows up at either end and is most rapidin the middle. This is advantageous with the observed characteristics ofapparatus of this nature, the operation tends to be more diificult outtoward the ends of the sweep range, since in certain cases the receivermay be operating on either side of the peak of the radio frequencyamplifying stages. It is accordingly a further object of the inventionto provide a scanning device for use in a receiver of this type in whichthe scanning voltage is generated by a multi-vibrator which generates inthe plate circuits a very high amplitude wave of substantially squareshape and filtering it with an enormous filter which pro- There maypossibly This type of .sine wave generator is very light and portableand has the further distinct advantage of requiring no moving parts suchas the rotors employed in several types of electric generators. tionwill become apparent to those skilled in the art as the descriptionthereof proceeds. For a better understanding of the invention, however.reference is made to the accompanying drawing in which:

The single figure is aschematic circuit diagram of a radio receiverembodying the invention.

Referring to the drawing, the invention is shown as applied to a radioreceiver of. the superheterodyne type comprising a tunable radiofrequency amplifier l whose input terminals are connected to a receivingantenna circuit 1. a first detector'or mixing device 3, an intermediatefrequency amplifier 4, a second detector 5, audio amplifier 6, loudspeaker I and automatic volume control device 8 which controls theoutput level of the receiver in a known manner. The aforementionedreceiver parts may beof any known type and a detailed descriptionthereof is not necessary to an understanding of the present invention.The beat frequency is supplied to the mixing device 3 by an oscillator 9whose frequency is controlled in a manner to be explained later by areactance control device Ill which is connected to a line H, which isalso connected to ground through a large condenser Cl and to a scanningoscillator l2 and also to a director device l3. Suitable operatingvoltage is supplied to the director device II from the intermediatefrequency amplifier 4 through a lead I.

Still referring to the drawing for a more detailed description ofcertain features of the invention and wherein the radio frequencyamplifler I is shown as comprising a circuit including a condenser l5and coil l6 which may be permeability tuned to pick up-desired carrierfrequencies as indicated at 11, as by means of a powdered ferro-magneticcore. The director unit l3 consists of a driver tube V." whose innercontrol grid is connected through lead M to the output of theintermediate frequency amplifier l,

duces a very close approximation to a sine wave.

and whose plate circuit comprises the low impedance primary windings l8and I9 which are coupled respectively to the secondary transformerwindings of the circuits 'ICl, T02, one of which is tuned to a frequencyof from 5 to lo kilocycles above the intermediate frequency, and theother of which is tuned by the same amount below the intermediatefrequency. These circuits feed their diodes VT! to develop thedifferential voltage across their resistors R2 and RI. As shown, one endof the resistor RI is grounded and one end of resistor R2 is connectedby lead H to apply the director voltage to the inner grid of theoscillator control tube VT. 1

The scanning device comprises the oscillator I! which preferablygenerates a current of a low audio frequency, the frequency of one-halfcycle per second having been found to be very suitable for the desiredpurpose. This oscillator comprises the tubes V' 15 and VTG, the plate ofone tube being connected to the grid of the opposite tube through thecondensers Cl and C2 having 9. capacity of one-half mfd., and the gridsof the tubes being connected to their cathodes through the 2.0 megohmresistors R5 and R6, to thereby produce a half cycle sweep voltage. Thisvoltage is supplied to the lead ll through condenser C3 which blocks offthe direct current, and the re- Still further objects of the invensistorR1, so that the sweep oscillator voltage and the director voltage aresupplied in parallel through the resistor R9 to the inner grid of thecontrol tube VTI. It will be understood, however, that it is notnecessary to modify the transconductance of the control tube by applyingthe director voltage to the inner grid as it may equally well be appliedto another grid of the tube as, for example, the screen grid or thesuppressor grid. Or alternatively, the director voltage may be appliedto the inner control grid while the sweep oscillator voltage is appliedto an outer grid, such as the suppressor grid.

It has been found that in receivers of this type, and especially whendesigned for the reception of carrier frequencies of the order of 100megacycles, especial attention must be given to the design of theoscillator circuit. Experience has shown that the cathode of theoscillator tube, herein shown as VT3, must be grounded in order toeliminate heater-to-cathode cyclic effects, when the source of heatervoltage is an alternating current or a direct current from the batterywhich also supplies a motor generator or vibrator type of plate supplyvoltage. At these high frequencies the grid of the tube has extremelypoor admittance effects, and if attempts are made to tune the gridcircuit, conditions almost invariably occur where the system willcommence oscillating at a parasitic frequency determined by theconstants of the plate circuit. It is, therefore, preferred to use anoscillator whcse plate circuit is tuned, and a tickler circuit providesa grid voltage of the proper phase to generate the oscillations. Theresonant plate circuit may have lumped constants comprising the coil Liand condenser 22, as shown, or may comprise a quarter-wave line orsomemodification thereof, such as two nested cups. Where the constantsof the circuit are lumped, a variable condenser is unsuitable for tuningdue to the varying L-C ratio, which gives much more stability at one endof the tuning range than at the other, thereby providing greatlyexaggerated variations over the range of sweep of the control tube. Bymaking the condenser 22 of a fixed value and varying the inductance ofcoil Ll so as to change its permeability, as by means of a powderedferromagnetic core, as indicated at 23, substantially equal percentagetuning effects are secured throughout the band of frequencies. Anadditional advantage of effecting the tuning of the oscillator by changeof permeability lies in a more simplified switching procedure wheredifferent coils are used for the reception of different frequencyranges. Each such coil may have attached to it permanently its owntuning condenser, thus forming a circulating path of low resistance; andthe only switching required being the switching connections at the gridsand plates of the tubes.

' It is found that the ordinary hydrogen reduced iron cores are not atall suitable for permeability tuning at frequencies of the order of 100megacycles. However, it has been found that ferromagnetic cores knownunder the trade name Aladdinite" are very satisfactory. These cores aremade from a synthetically produced ferromagnetic mass powder consistingsubstantially wholly of magnetic oxide of iron in the form of minuteparticles, substantially all of which, as they appear under amicroscope, are of generally rounded form. These particles arepreferably to that. of the plate.

molded into cores by mixing with several per cent of Bakelite as abinder.

The grid of the oscillator VIIi is shown as being energized throughcondenser C6 and coils L3 and L2 connected in series. Since the accuracyof calibration of a superheterodyne receiver depends largely on theoscillator, stable oscillators are, therefore, highly desirable. It isalso highly desirable that there be a 180 phase shift between the plateand grid voltages of the tube, and, where a tickler coil is used toprovide the feed-back, this is always given a polarity such as toreverse the phase of the grid relative The mutual inductance oi. thetickler coil with coil LI of the tank circuit is designated as L2 andits leakage inductance by the reference character L3. Depending upon thephysical placement of the component parts, the length of leads employedand numerous other factors, the ratio between these two inductancesLil-L2, as well as their actual values, may vary between wide limits.

A desirable oscillator for use with a control tube in a system of thistype would be one without phase angle, that is, one in which the plateenergy exactly reinforces the grid energy with no reactive component.This ideal condition would require a vacuum tube which is a simplegenerator of negative resistance, an arrangement which may be closelyapproximated in practice. If the oscillator is well designed withrespect to this feature, its plate supply voltage may vary within ratherwide limits without substantially affecting the frequency of theoscillations generated, although their amplitude will be affected tosome extent. With respect to the requirements of the oscillator controltube, these are exactly the opposite. It is desired that this generatepure reactance only and neither negative nor positive resistance. It isnecessary, therefore, that the control tube be coupled into the tubecircuit to be controlled in such manner that great phase shifts aresecured either in the energy feed to the grid or the energy of the platecircuit, or both.

A standard means of shifting phase is to place a reactance in serieswith a resistance and take the voltage drop across the reactance. Inpractice, only a limited phase shift can occur, since a shift wouldrequire an infinite resistance a and a perfect reactance. In practice,approximations of 90 can be reached as a limit, but since in .radiofrequencies only relatively small resistances can be employed, it isonly practical to secure a 60 or 70 phase shift instead of the desirable90". A further phase shift, however, may be secured by connectinganother phase shifting network in series with the first, the secondnetwork producing an additional phase shift approximating 90. Since thetwo phase shifting means in cascade have the possibility ofapproximating an phase shift, it is possible to utilize a portion ofeach in any desired combination to give a resultant phase shift of 90,which, of course, represents a pure reactance.

Now considering the circuit which is grounded at the lower end of coilLl, the grid receives charging current through L2 representing themutual inductance of the tickler coil with Li. The grid current at theend of L2 is thus exactly 180 out of phase with the circulating currentin Ll. Before reaching the grid of tube VT3,

the current must pass'through some series leakage inductance,represented by L3, which produces a reactance of rather small amount,but

phased in relation to that of the plate tank circult as to cause thetube to oscillate. The amount of phasing may be varied as desired bymaking L3 of the proper value, even, if necessary, by winding anadditional uncoupled coil in' series with the grid, or, by more looselycoupling L2, and by making this coil larger we can in- .crease theleakage inductance to any desired amount of without having any physicalcoil, as represented by L3. With this energy, preliminarily phased, asdescribed, the grid energy of tube VT3 may now be further phased in thesame direction by means of the circuit comprising resistor R8 and thegrid-cathode capacity of the control tube VT. It will be understood thatcondensers C5 and C8 are merely for the purpose of blocking the directcurrent and are 'so large that the phasing effect is negligible.

By a suitable choice of the values of L3. and R8 it is possible toproduce the desired degree of reactance which we wish with a maximumstability of the oscillator and a minimum loading effect due to theconnection of the control tube VTl with the oscillator. Considerabledifficulty is met with in complicated networks of this type where theoscillator is required to supply voltage to the mixer device, such as 8,as well as having a control tube, such as VT, connected across it. Thesystem is prone to be so heavily overloaded as to cease oscillation,entirely, or if excessive amounts of negative resistance are generatedin the control tube, parasitic oscillations are likely to occur at afrequency determined by some loop, which may be almost anywhere in thesystem, or even coupled rather loosely to the circuit system. With thecircuit arrangement described, phase relations may be established sothat the control tube VTl acts substantially as a pure reactance devicein controlling the frequency of the oscillations generated by tube VT3.

Instead of securing the double phasing effect by the arrangementpreviously described, it may be secured in a somewhat satisfactorymanner by the inclusion of a resistor RH in series with the plate oftube V'Il and the tank circuit comprising coil Ll, in which thepreliminary phasing is secured by resistor R8 alone at the input of thetube and additional phasing secured in an additive sense by RH. In caseswhere it may be desired to eliminate the effect of resistor RH,

a suitable short circuiting switch may be provided therefor as indicatedat 24.

The control tube V'Il is provided with the usual supply voltage, itsinner grid being given a normal bias of approximately -6 volts byconnecting its cathode to the bleeder resistor 25 at a pointapproximately this number of volts above ground. This permits theoperation of the tube at the mid-point of its control range, asdetermined by plotting a curve of grid bias against oscillatorfrequency, which results in an S-shaped curve, so that we secureoperation about the mid-point thereof. This mid-point usually occurs atabout --6 volts and if this value is changed to -3 volts, the tubebecomes much more effective and if changed to -9 volts, the control tubebecomes practically ineffective. The control tube is then in conditionto respond to sweep. In director voltages which are in the form ofpositive and negative voltages of various amounts, the polarity of theapplied voltage determines the sense of direction of the control tubeand the amplitude thereof determines the amount of the control. Thevoltage generated by the sweep oscillator is arranged to be a total peakto peak of 6 volts so that the effective voltage at the grid of controltube VT from the sweep oscillator is :3 volts. This voltage swingsufllces to sweep the receiver through a range of frequenciesapproximating 1% of the carrier frequency. The director, including itsresonant circuits TC I and T02 and driver tube VTl, is so designed thaton the reception, of any usable signal strength it generates :20 voltseven on the weakest signal for which the system is designed, and if thesignal strength is increased by as much as 100 decibels, the directorvoltage will only increase by 5 volts. This limited characteristic, hereand at other portions of the receiver, serves to prevent the building upof high amplitude-surge voltages which might give rise to motor-boatingphenomena.

The director device is designed to generate a voltage approximately 7times greater than the control tube can usefully employ, namely, 3volts, since the higher the voltage generated by the director, thecloser to actual resonance the set will be automatically tuned.Furthermore, if the sweep oscillator encounters a carrier at the extremeend of its range, as for instance at 3 volts, the sweep oscillator cangenerate 6 volts from that point, which voltage must be overcome by thedirector and still leave a sufflcient margin of voltage to tune thereceiver to the incoming carrier within approximately 1 kilocycle, atthe carrier frequency of 150 megacycles.

Since receivers of this type must operate at points low down in thenoise level in making an utmost effort to pick up a carrier and securecontact with a transmitter, and since at any given instant there isequal distribution of noise throughout the spectrum, it is preferable toarrange the intermediate frequency amplifier and director system so asto have a symmetrical operation with respect to the center frequency.Otherwise more noise may be generated on one side than on the other anda small and fluctuating noise voltage will appear at the output of thedirector tube. Due to the fact that it only requires a few millivoltsapplied to the grid of the control tube VT4 to cover an entire channel,while the director supplies voltages of many times this amount,symmetrical operation about a center frequency serves to reduce thenoise component in the output of the receiver.

It has been noted after considerable experience with receivers of thistype that as the frequency is raised we can attain such a frequency thatthe ordinary noises which prove so troublesome in ordinary reception,such as static, diathermy machines, sparking commutator motors andignition noises, gradually die out, and at some vague point, probablybelow megacycles, become extremely rare in a sharp receiver. There isleft, of course, the thermal agitation noise in the circuits and thenoise and shot eifects in the tubes. This is a more or less regular hisstype of noise of quite a definite and dependable character and is easilyrecognized by anyone skilled in the art.

Where space and weight are no consideration in designing a receiver ofthis type, we should probably always prefer a definite sweep voltagesuch as that produced by the 0.5 cycle oscillator shown, but experiencehas shown that if the filter condenser C4 be lessened in value toemphasize the noise component in the output of the director connectedfrom the control tube and connected due to the amplified noise arisingfrom the thermal agitation in the circuit lS-Hi, that the 0.5 cycleoscillator can be omitted with rather good results. In this case thefluctuating noise voltages produced in the output of the director by theamplification of the thermal agitation and shot effect current of thefirst stage in the differential circuits TCI and T02 are sufficient tosweep the control tube through its entire range quite rapidly and atrandom, but in a given period we find we cover the entire useful rangeof sweep in less than a second, although it is recognized that severalpulses of the same polarity may follow each other rather than the smoothpulses of always alternate polarity which we obtain from the moredisciplined action of the 0.5 cycle oscillator. The above conditionobtains only when the receiver is of great sensitivity, 1. e.,suflicient to develop on peaks a voltage sufllcient to fully control thecontrol tube.

The tuning ranges of receivers of the type described are of two distinctclassifications: first,

for comparatively large tuning ranges that are to be covered, whereoperation may be expected on any one frequency throughout these largeranges; and, second, where specific point-to-point service might be setup on an assigned frequency. In this latter case, no tuning range at allin the ordinarily accepted sense is required, but merely excursionaround the assigned frequency to establish contact between transmitterand receiver at will. In this latter case probably much closertolerances can also be expected with respect to the accuracy of thetransmitter frequency, as it would probably be crystal controlled, andonly the receiver errors would remain. We thus have the two conditions,one a large tuning range and a small sweep range; the other, no tuningrange at all, but a sweep range probably less than 1%. In a receiverdesigned for operation over a tuning range of from 100 to 200 megacyclesthe following circuit constants are found suitable.

Vacuum tubes of the following types are found suitable for use'with thecircuits described:

VTI=type 6J7 pentode VT2=type 6H6 double diode VT3=type 955 acorn triodeVT4=type 954 acorn pentode VT5, VT6=type 6N7 double triode In using thereceiver, the operator tunes the input circuit of the radio frequencyamplifier l so that it will pass the carrier frequency or frequencies ofthe transmitter or transmitters with which it is desired to establishcontact. In the illustrated embodiment this tuning is effected by manualadjustment of the permeability tuning means I1. Since the frequency ofoscillator VT3 is being cyclically varied through the action of thecontrol tube VT4 whose reactance is in turn being cyclically varied bythe sweep oscillator VT5, VTG, as soon as a carrier appears which formsa beat frequency with the oscillator frequency which lies within thefrequency band passed by the intermediate frequency amplifier 4, anenergizing voltage is applied through lead H to the control grid of thedirector driver tube VTI. If the output of the director VT! is disto avoltmeter and a signal is tuned through, a voltage of or -20 volts isgenerated in the director when the receiver is 5.0 kc. ofi tune, inother words, when the intermediate frequency impressed on the directoris 5 kc. above or below the mid-intermediate frequency. This voltage isdeveloped for all signal strengths down to the noise level of thereceiver, which is equivalent to a voltage of approximately 1 microvolton the grid of the first radio frequency amplifier l and is preventedfrom rising substantially above this value for stronger signals by thelimiting action of driver tube VTI.

Tube VT4 is an acorn type 954 tube and has a characteristic curve suchthat at a plate voltage of 150 volts, its maximum Gm. is at about -3volts on its control grid while its Gm. becomes negligible at about --9volts on its control grid. The maximum output voltage of the directoris, therefore, many times greater than that required to fully controlthe control tube VTI and gives a steep slope of output voltage vs.frequency change so that a departure of only 1 kc. from themid-intermediate frequency will develop 3 volts of appropriate polarityto tune the oscillator VT3 toward the correct frequency.

Should a carrier appear when the voltage of the grid of control tube VT4is increasing and is, for example, at 8 volts and which would be tunedin when the grid voltage of VT is at 4 volts, as the'grid voltage of VTpasses successively through 7, 6 and -5 volts under the control of theoscillator I2 as the tuning position is being approached, nointermediate frequency appears in the output of the amplifier 4 and,therefore, no energizing voltage is developed by the director VTZ. Asthe oscillator l2, however,

causes the grid voltage of tube VTl to closely approach 4 volts, thefrequency of oscillator VT3 is increased, an intermediate frequencyvoltage of increasing value appears in the output of the amplifier 4,which voltage energizes the director VTZ and a voltage appears in thelead H which tends to increase the voltage of the grid of VT towards thedesired voltage of -4.0. In other words, as the oscillator frequencyapproaches the proper tuning in frequency, the voltage generated by thedirector assists the voltage output of the oscillator I! in causing thefrequency of the oscillator VT3 to approach the correct tuning-in value.As the voltage of the grid of W4 is further increased by both theoscillator l2 and director VT! it will, of course, pass through thevalue of 4.0 volts when the frequency of oscillator VT! is such as toproperly tune in the station. At this instant since the mid-intermediatefrequency is being impressed upon the director VTZ, the output voltageof the director is zero. An instant later, however, the voltage of thegrid of VT is increased to a value slightly above 4.0 volts by theoscillator I2 and the frequency of the oscillator VT3 is correspondinglyincreased so that a frequency above the mid-intermediate frequencyappears in the output of the amplifier '4 and a voltage is generated bythe director VT! which opposes the increasing voltage being generated bythe oscillator l2. This opposing voltage is of sufllcient amount tocounterbalance the increase in voltage and to maintain the voltage ofthe grid of VT4 at a value only slightly above 4.0 volts, which value issuch as to maintain the oscillator VT3 tuned within 1.0 kc. of thecorrect. frequency. These opposing voltages are balanced withinextremely close limits, since to tube the receiver entirely through astation 10 kc. wide requires a change in voltage on the grid of thecontrol tube of only 0.04 volt for a carrier frequency of 150,000 kc. Asthe voltage generated by the oscillator l2 gradually passes up to thelimit voltage of 3 volts and returns in the opposite direction to avoltage just above 4.0 Volts, these voltage changes have small influenceon the grid of tube VT! and, therefore, do not appreciably change thefrequency of the oscillator VII. When the voltage generated byoscillator l2 reaches the value of 4.0 volts, the oscillator VT! is nowgenerating the correct frequency and the output voltage of the directorVT! passes through zero value. As the output voltage of oscillator I2decreases to a value only slightly below 4.0 volts, a voltage appears inthe output of the director VT2 which rises sharply and which opposes anydecrease in voltage in the lead H by the oscillator l2 so that thedirector voltage impressed upon the lead II is sufllcient tosubstantially overcome the decreasing voltage applied to this lead bythe oscillator i2 and to prevent the grid voltage of control tube VTIfrom decreasing below the desired value of 4 .0 volts by any substantialamount so that the frequency of oscillator VT! is maintained at a'propervalue to maintain the station tuned in. As the output voltage ofoscillator l2 passes successively through the values 5, 6, 8 and itslimiting value of --9 volts, the net voltage impressed on the lead IIand the control grid of tube V'Il suffers no substantial change but ismaintained by the director VT2 at a value only slightly less than 4.0volts and the receiver is thus maintained in tuned-in conditionthroughout the full range of sweep voltage of the oscillator l2. Thereceiver remains locked on the carrier even though the carrier frequencyshould vary considerably. Upon interruption of the carrier, the scanningaction of the oscillator VT! is resumed under the control of the sweeposcillator VTS-VTG since the director device VT! is no longer generatingany director voltage to operate the control tube VTl. Upon reappearanceof the carrier, the receiver will again lock on it in the mannerdescribed, should its transmitter be the only one operating. However, iftwo or more transmitters are operating on different carrier frequencieswithin the acceptance band of the input circuit l-l6, the receiver willautomatically lock on the first carrier passed over, since theoscillator VT3 will produce a difference frequency with such carrierfrequency which lies within the acceptance band of the intermediatefrequency amplifier 4.

While the receiver shown in the drawing and above described is a singlesuperheterodyne, the apparatus in which the invention was developed wasa double detection type superheterodyne with a first intermediatefrequency of 17.5 megacycles and a final intermediate frequency of 0.46megacycle. The term inertialess" as used in the following claimsdesignates a control means having no moving mechanical parts and hencefree of mechanical inertia.

I have described what I believe to be the best embodiments of myinvention. I do not wish, however, to be confined to the embodimentsshown, but what I desire to cover by Letters Patent is set forth in theappended claims.

I claim:

1. A radio receiving system for receiving carriers of differentfrequencies associated with dif- 7 ferent communication channelscomprising in combination. a circuit tunable over a wide range offrequencies, means comprising a reactance generating device for tuningsaid circuit over a range of frequencies at least as wide as thatconsisting of a plurality of adjacent communication channels, means freeof mechanical inertia connected to said reactance generating device andarranged to automatically generate oscillations of low frequency and ofsufllcient amplitude to repeatedly change the reactance of saidreactance generating device by an amount suiiicient to cause the tuningof said circuit through said wide range of frequencies and frequencyresponsive means responsive to the reception of any one of the carriersfor maintaining the tuning of said circuit approximately at a desiredfrequency within said range of frequencies.

2. A radio receiver comprising an amplifier having an input circuithaving an acceptance band at least as wide as the frequency spectrum ofat least three transmitting stations operating on different carrierfrequencies, a tunable circuit, means free of mechanical inertiaarranged to automatically tune said tunable circuit over a range offrequencies difierent from but approximately as wide as the acceptanceband of said input circuit, a mixing device connected to the output ofsaid amplifier and to said tunable circuit, and a frequency responsivedevice connected to the output of said mixing device and arranged toautgmatically render ineffective the action of said automatic tuningmeans on the tuning of said tunable circuit in response to theimpression on said input circuit of carrier current from any one of thetransmitting stations.

3. A radio receiver as claimed in claim 2 in which the means forautomatically tuning the circuit comprises a low frequency oscillatorarranged to generate successive oscillations of constant amplitude, saidoscillator comprising a pair of similar electron discharge tubes andmeans for coupling the plate of each tube to the grid of the other tube.

4. A radio receiver comprising an input circuit having an acceptanceband at least as wide as the frequency spectrum of a plurality oftransmitting stations having different carrier frequencies, a tunablecircuit, means free of-mechanical inertia for continuously andautomatically tuning said tunable circuit over a range of frequenciessubstantially as wide as the acceptance band of said input circuit, amixing device connected to said input and tunable circuits and afrequency responsive device connected to the output of said device andarranged to render ineffective the action of said automatic tuning meanson the tuning of said tunable circuit in response to currents in theoutput of said mixing device of a predetermined frequency.

5. In a radio receiver of the superheterodyne type tunable to receivesignaling currents of different carrier frequencies within a wide bandof frequencies and each identified with a different transmittingstation, said receiver comprising an oscillator and an intermediatefrequency amplifier, a mixing device connected to the input of saidintermediate frequency amplifier and to said oscillator, a circuitnetwork connected to the input of said mixing device and having anadmittance band sufllciently wide to transmit at any instant carriercurrents from a plurality of the transmitting stationsof means free ofmechanical inertia electrically connected to said oscillator andarranged to automatically vary the frequency of the currents generatedby said oscillator throughout a range of frequencies substantially aswide as said band, a director device coupledto the output circuit ofsaid intermediate frequency amplifier and responsive to the frequency ofthe currents passing through said amplifier and a reactance generatingdevice connected to said director device and arranged upon theenergization thereof by said director device to maintain the frequencyof the currents generated by said oscillator substantially constant.

6. In combination with a radio receiver of the superheterodyne typeadapted to receive ultra high frequency carrier currents of difierentfrequencies and comprising a first detector and an oscillator, saidoscillator comprising a vacuum tube having coupled grid and platecircuits, said plate circuit comprising a coil and a fixed condenserconnected in parallel and providing lumped constants determining theresonant frequency of said plate circuit, control means for adjustingthe inductance of said coil to cause said oscillator to generatecurrents of a predetermined frequency and means free of mechanicalinertia for automatically causing the frequency of the oscillator tovary progressively from a first frequency to a second frequency and fromthe second frequency to said first frequency a plurality of times insuccession, said first and second frequencies being adapted to combinewith the highest and lowest received carrier frequencies to form a beatfrequency.

7. A radio receiver adapted to receive signal currents of differentcarrier frequencies lying within a band of frequencies higher than 20megacycles and the frequency difference between the highest and lowestfrequency signaling currents being of the order of 200 kilocyclescomprising, in combination, inertialess means arranged to automaticallyand cyclically tune said receiver over the frequency band, meansresponsive to the reception of any one of several carriers having afrequency within said band for rapidly and automatically rendering saidinertialess means ineffective to continue the tuning of the receiverover said band and to automatically permit the cyclic tuning of thereceiver by the said inertialess means again, in response to thedisappearance of the carrier.

8. A radio receiving system comprising in combination, an input circuithaving an acceptance band at least as wide as the vfrequency spectrum ofa pluralii ,7 of transmitting stations of different carrier frequencies,an electron discharge tube amplifying device having its input connectedto said input circuit and arranged to amplify to a very high degree thethermal agitation noise appearing in the input circuit, a tunablecircuit connected to said amplifying device, a reactance generatingdevice connected to said tunable circuit, a frequency responsivedirector device coupled to the output of said amplifying device and afilter circuit connecting the ouput of said director device to saidreactance generating device, the time constant of said filter circuitbeing so small as to cause the amplified noise voltage variationsappearing in the output of said director device to vary the reactance ofthe reactance generating device in such a manner as to cause the randomtuning of said tunable circuit over a wide range of frequencies.

9. A radio receiving system comprising in combination, an input circuithaving an acceptance band at least as wide as the frequency spectrum ofa plurality of transmitting stations having

